Add that to the list then... firefish I was told were 50gal+ (though a cursory Google search still says 20+?). They were one of my first considerations, really cool fish. Trying to figure out what to do about a lid without my HOB filter getting in the way and without blocking the light.Those guys are badly informed, then. There's some debate about some of the more common nano fish being suitable or not, but there are a good few tiny fish that are definitely fine. There are also a couple of blenny species that pick a hole and stay in it- barnacle blennies, the sailfin blennies KP Aquatics has. You could practically keep them in a breeder box permanently if you could ensure that they had something in it that they wanted to sit in.
A firefish would be a good option. They can be a bit timid, but should hover visibly when not spooked, and they have pretty colors and a charming, twitchy dorsal fin. They're best added first due to that timid nature. They do, though, need a very tightly fitted lid. Everything mobile (including shrimp!) needs a lid, but firefish will very quickly show you why a lid is important if kept in a lidless tank.
I honestly think this LFS is just trying to pull one over on me. They conveniently didn't tell me any of this until after I purchased the rock and sand (which, like most of their inventory, is explicitly nonrefundable), before which I established multiple times it was only 10gal. They also said no more than 3 corals of any kind, ever. After pocketing my money they suddenly want me to upgrade to 30gal or more. I call BS